redefine VERIFY_CHECK to empty in production (non-VERIFY) mode

As suggested in issue #1381, this will make things simpler and
improve code readability, as we don't need to force omitting of
evaluations on a case-by-case basis anymore and hence can remove
lots of `#ifdef VERIFY`/`#endif` lines (see next commit). Plus,
VERIFY_CHECK behaves now identical in both non-VERIFY and coverage mode,
making the latter not special anymore and hopefully decreasing
maintenance burden. The idea of "side-effect safety" is given up.

Note that at two places in the ellswift module void-casts of return
values have to be inserted for non-VERIFY builds, in order to avoid
   "variable ... set but not used [-Wunused-but-set-variable]"
warnings.

src/util.h

@@ -132,15 +132,12 @@ static const secp256k1_callback default_error_callback = {
 } while(0)
 #endif
 
-/* Like assert(), but when VERIFY is defined, and side-effect safe. */
-#if defined(COVERAGE)
-#define VERIFY_CHECK(check)
-#define VERIFY_SETUP(stmt)
-#elif defined(VERIFY)
+/* Like assert(), but when VERIFY is defined. */
+#if defined(VERIFY)
 #define VERIFY_CHECK CHECK
 #define VERIFY_SETUP(stmt) do { stmt; } while(0)
 #else
-#define VERIFY_CHECK(cond) do { (void)(cond); } while(0)
+#define VERIFY_CHECK(cond)
 #define VERIFY_SETUP(stmt)
 #endif